1. Field of the Invention
The present invention relates to an alloy for use in making of golf clubs, particularly to an alloy with low density, high ductility and high resistance to corrosion.
2. Description of Related Art
An alloy is a mixture of metals, such as a metal mixed with additions of metals or sub-metals for various special purposes. When a metal is mixed with other metals or sub-metals, its mechanical properties, such as the melting temperature, strength, ductility, electrical resistance, thermal conductance, heat treatment properties, resistance to corrosion and magnetic properties are all promoted.
A set of golf clubs generally comprises woods, irons, pitching wedges, sand wedges, putters, etc. The iron club has a shorter striking distance but gives better good controllability and a higher striking height than the wood club has. In recent years, the iron club has been designed to have a hollow club head in order that the iron club may possess the advantages of the wood club.
With reference to the table in FIG. 1, two manufacturing methods of the head of the golf club are listed; one of them is precision lost-wax casting and the other one is forging. Besides the methods listed in the table of FIG. 1, some iron club heads are finished by surface plating, such as nickel-plating, cobalt-plating, etc. and paneling. Among these methods, the method of the precision lost-wax casting has the lowest manufacturing cost, however the method of the forging has more advantages than the method of precision casting, which can be seen from the comparison in the table of FIG. 1. The mechanical properties of the precision lost-wax casting and the forging are listed in the table of FIG. 2.
The major object of the designing of the golf club is to improve the controllability and stability of striking via good striking points, and the designing has following tendencies:
1. the heads of the clubs are enlarged in order to increase sweet spots and the probability of successful striking; the volume of the woods can be from 280 cc to 310 cc, and even to 350 cc, and the irons also have some oversized features.
2. the center of gravity of the club head is lowered in order to obtain a very stable striking of the ball, good striking points and long striking distance.
3. the shape of the club head is designed to have a strengthened club face with low air drag.
Since the club heads have a variety shapes, an alloy metal is a popular material for manufacture thereof, particularly an alloy which combines high strength with high ductility and resistance to corrosion. However, the alloys which are used to make club head at present do not satisfy all the requirements of the club head. For example, titanium alloyed with stainless steel has good resistance to corrosion from a damp or salty atmosphere, however its ductility and impact value are not good enough; the 304 stainless steel has an elongation of 40%xcx9c60%, however its strength is not enough. The S25C with a tensile strength of 75 ksixcx9c85 ksi and an elongation of 30%xcx9c35% is the best material for use in forging of a club head, however, its resistance to corrosion is a little insufficient.
The research of the golf materials shows that if an alloy for heads of golf clubs has low density, high ductility and toughness, then the head of the club may be designed with a larger volume, and also the controllability and striking stability of the club will be increased. Presently, manufacturers of golf clubs have a common opinion that the best alloy for the golf club irons should have a tensile strength about 80 ksi to 120 ksi, which is 1.0 to 1.5 times of the tensile strength of the soft iron used for forging, an elongation over 40% and the higher the better, a density below 7.9 g/cm3, and a good resistance to the corrosion.
It has been found that mechanical properties can be promoted by controlling the contents and by performing heat treatment to obtain high strength and toughness, good resistance of low or high temperature, and resistance to the corrosion. The following papers have described these characteristics in detail.
xe2x80x9cthe Structure and Properties of Austenitic Alloys Containing Aluminum and Siliconxe2x80x9d by D. J. Schmatz, Trans. ASM., vol. 52, p. 898, 1960; xe2x80x9cPhase Transformation Kinetics in Steel 9G28Yu9MVBxe2x80x9d by G. B. Krivonogov et al., Phys. Met. and Metallog, vol. 4, p. 86, 1975; xe2x80x9cAn Austenitic Stainless Steel Without Nickel or Chromiumxe2x80x9d by S. K. Banerji, Met. Prog, p. 59, 1978; xe2x80x9cPhase Decomposition of Rapidly Solidified Fexe2x80x94Mnxe2x80x94Alxe2x80x94C Austenitic Alloysxe2x80x9d by J. Charles et al., Met. Prog., p. 71, 1981; xe2x80x9cDevelopment of Oxidation Resistant Fexe2x80x94Mnxe2x80x94Al Alloysxe2x80x9d by J. Garcia, et al., Met. Prog., p. 47, 1982; xe2x80x9cNew Stainless Steel Without Nickel or Chromium for Alloys Applicationsxe2x80x9d by R. Wang, Met. Prog, p. 72, 1983; xe2x80x9cAn Assessment of Fexe2x80x94Mnxe2x80x94al Alloys as Substitutes for Stainless Steelxe2x80x9d by J. C. Benz et al., Journal of Metals, p. 36, 1985; xe2x80x9cNew Cryogenic Materialsxe2x80x9d by J. Charles et al., Met. Prog, p. 71, 1981; xe2x80x9cTEM Evidence of Modulated Structure in Fexe2x80x94Mnxe2x80x94alxe2x80x94C Alloysxe2x80x9d by K. H. Ham, Scripta Metall, vol. 20, p 33, 1986; Electron Microscope Observation of Phase Decompositions in an Austentic Fe-8.7 Al-29.7 M-1.04 C Alloyxe2x80x9d by S. C. Tjong, Mater. Char, vol. 24, p. 275, 1990; xe2x80x9cGrain Boundary Precipitation in an Fe-7.8 Al-1.7 Mn-0.8 Si-1.0 C Alloyxe2x80x9d by C. N. Hwang et al., Scripta Metall, vol. 28, p109, 1993; xe2x80x9cHot-Rolled Alloy Steel Platexe2x80x9d by T. F. Liu U.S. Pat. No. 4,968,357, 1990.
Reviewing the above noted references, it can be found that in the Fexe2x80x94Alxe2x80x94Mnxe2x80x94C based alloys, manganese content is added to stabilize the austenite structure and retain an FCC structure under a room or lower than room temperature, which is beneficial to enhance the workability and ductility of the alloy. An aluminum content has a strong effect on oxidation resistance. A carbon content mainly helps precipitation of strengthening elements when the alloy is quenched rapidly after a solution heat treatment at a temperature from 1050xc2x0 C. to 1200xc2x0 C., and then aged at a temperature from 450xc2x0 C., to 750xc2x0 C. The alloy has a mono austenite structure during the quenching, and the fine (Fe, Mn)3AlCx xcexa carbides are precipitated coherently within the austenite matrix during the aging. Additionally, after a lengthy aging, phase decomposition like xcex3xe2x86x92xcex1+xcex2-Mn or xcex3xe2x86x92xcex1+xcex2-Mn+xcexa is produced on the grain boundary of the alloy dependent on its chemical composition. The coarse precipitates of xcex2-Mn will deteriorate the ductility of the alloy. Consequently, to obtain carbides precipitated coherently within the austenite matrix and without the coarse xcex2-Mn being precipitated therein is an important method for the alloy to possess a satisfactory strength and ductility.
It is found that the Fexe2x80x94Alxe2x80x94Mn based alloys mainly consisting of iron, 5 to 12 wt % aluminum, 20 to 35 wt % manganese, and 0.3 to 1.3 wt % carbon, and after being solution heat treated, quenched and aged, will have different mechanical properties dependent on their chemical compositions, the tensile strength has a range of 80 ksi to 200 ksi, the yield strength has a range from 60 ksi to 180 ksi and the elongation has a range from 62% to 25%. As shown in the tables of FIG. 3 and FIG. 4, the chemical compositions and mechanical properties of the typical Fexe2x80x94Alxe2x80x94Mn alloys, which have been studied by experts in this field, are listed for comparison.
The inventor has worked on the analysis and study of the Fe-10 wt %, Al-30 wt %, Mn-1 wt %, C alloy and the Fe-8 wt %, Al-30 wt %, Mn-0.8 wt %, C alloy. The study proves that after being heat treated at a temperature of 1100xc2x0 C. for 0.5 to 2 hours, the Fe-10 wt %, Al-30 wt %, Mn-1 wt %, C alloy has its hardness value from Hrb 82.7 to 88.9, tensile strength from 111 ksi to 124 ksi, yield strength from 79.7 ksi to 97 ksi, elongation from 58.9% to 63.3%, the Hall-Petch relationship between the tensile strength ("sgr") and the grain size (d): "sgr"=68.72+21.2xc3x97dxe2x88x920.46, a metallograph as shown in FIG. 5, and an unsatisfactory resistance to the air corrosion after having been tested for 48 hours by exposure to salt spray. Other experts have also studied to prove that after being forged at temperatures from 1050xc2x0 C. to 1200xc2x0 C., the Fe-10 wt %, Al-30 wt %, Mn-1 wt %, C alloy has a surface roughness of Ra=3.1 to 5.9 xcexcm, and a metallograph as shown in FIG. 6. After being heat treated at a temperature of 1100xc2x0 C. for 0.5 to 2 hours, the Fe-8 wt %, Al-30 wt %, Mn-0.8 wt %, C alloy has its tensile strength from 111 ksi to 120 ksi, yield strength from 71.1 ksi to 8301 ksi, elongation from 58.5% to 64.7%, the Hall-Petch relationship between the tensile strength ("sgr") and the grain size (d) is "sgr"=68.72+21.2xc3x97dxe2x88x920.46, and an unsatisfactory resistance to air corrosion after having been tested for 48 hours by exposure to salt spray. Further experts have also studied to prove that after being forged at temperatures between 1050xc2x0 C. to 1200xc2x0 C., the Fe-8 wt %, Al-30 wt %, Mn-0.8 wt %, C alloy has a surface roughness of Ra=3.2 to 5.7 xcexcm.
The characteristic of the invention is to produce an alloy for a head of a golf club by suitable addition of alloying elements and by controlling a heat treatment condition. The alloy of the invention has a low density (density within 6.78 to 7.05 g/cm3), a high ductility (elongation above 65%), a tensile strength within 80 ksi to 120 ksi, a yield strength within 55 ksi to 70 ksi and high resistance to corrosion via humidity. In accordance with the present invention, the mechanical properties of the alloy for heads of golf clubs are different to those of the other recently developed alloys and more in conformity with the requirement of high strength, high ductility and resistance to corrosion of the heads of golf clubs.
The object of the present invention is to provide a low density and high ductility alloy for making a golf club head, the alloy consisting essentially of 25 to 31 wt % manganese, 6.3 to 7.8 wt % aluminum, 0.65 to 0.85 wt % carbon, and 5.5 to 9.0 wt % chromium, and the balance being iron. Addition elements 0.8 to 1.5 wt % silicon, 2.0 to 5.0 wt % titanium, or 0.5 to 1.0 wt % molybdenum are optionally added to the alloy of the invention. Due to the addition of chromium, titanium and molybdenum, the alloy of the invention has a good resistance to corrosion. A good finished surface quality is obtained after the alloy is forged at a temperature from 800xc2x0 C. to 1050xc2x0 C. Furthermore, a combination of high ductility and high tensile strength is obtained after the alloy has been treated at a temperature from 980xc2x0 C. to 1080xc2x0 C. for 1 to 24 hours. Therefore the alloy with low density, high strength, high ductility, good resistance to corrosion, and a good surface finish quality is obtained to satisfy the requirements of the heads of golf clubs.